I've currently got Little Snitch on my Mac OS X which work very well for blocking most things except recently I've noticed my web access slowing down or even hanging altogether on particular websites.

When it does slow down I can quickly see in the browser which domain it is and I simply add it as a rule to block. Everything works great and all website now seem to load quicker too - but it's annoying to do that every time and not only that but the uneasy feeling I get that somebody's actively watching you all the time!

Is there a Mac tool out there that can simply block top level domains?

This OSX Daily article suggest a neat trick where you add the relevant domain to your hosts file and then point it to 127.0.0.1. Nice way to ensure the traffic won't go anywhere but in.

Another alternative would be to take an old machine and set up your own ipCop or Smoothwall firewall with Squid installed. Set this as a transparent proxy and then block the top level domains in Squid. You may even consider doing this in a Virtual Machine and letting your machine connect through the VM instead of directly. Will be tricky to configure but should work well.